/Engine/Plugins/Experimental/GeometryScripting/Source/GeometryScriptingCore/Public/GeometryScript/MeshMaterialFunctions.h
This populates the MaterialIDList with Material IDs for each triangle in the TriangleIDList. If a triangle is not present in the Target Mesh the number -1 will be used for the corresponding Material ID. If Material IDs are not enabled on the TargetMesh no Material IDs will be added to the result list.
static [UDynamicMesh](API\Runtime\GeometryFramework\UDynamicMesh) * GetMaterialIDsOfTriangles ( [UDynamicMesh](API\Runtime\GeometryFramework\UDynamicMesh) * TargetMesh, [FGeometryScriptIndexList](API\Plugins\GeometryScriptingCore\GeometryScript\FGeometryScriptIndexList) TriangleIDList, [FGeometryScriptIndexList](API\Plugins\GeometryScriptingCore\GeometryScript\FGeometryScriptIndexList) & MaterialIDList, [UGeometryScriptDebug](API\Plugins\GeometryScriptingCore\GeometryScript\UGeometryScriptDebug) * Debug )